Last time we talked about dark energy and its equation-of-state parameter, w. This number tells you how quickly the dark energy density changes as the universe expands; if w=-1, the density is strictly constant, if w>-1, the density decreases, and if wa is the scale factor describing the relative size of the universe as a function of time, then the density goes as a-3(1+w).) For comparison purposes, cosmological "matter" (slowly-moving massive particles) has w=0, and "radiation" (relativistic particles, including photons) has w=1/3.

Einstein's cosmological constant is just the idea that there is a fixed minimum energy density everywhere in the universe; this vacuum energy would correspond to w=-1. It's easy enough to get an energy density that slowly diminishes, with w>-1; all you need to do is invent some scalar field slowly rolling down a very gentle potential, so that the energy is nearly constant but in fact gradually diminishes.

What about ww would exclude ww using supernova data is the one by Garnavich et al., the High-Z Supernova Team. I am friends with these guys -- Brian Schmidt, leader of the collaboration, was my officemate during grad school -- and one day they called me up to ask whether there was a good reason why they could ignore wDominant Energy Condition (DEC), which is what allows you to prove that energy can't propagate faster than the speed of light. So I pointed out that imposing the DEC would exclude the wwMost people, anyway. A notable exception was Robert Caldwell at Dartmouth, who wrote a paper suggesting that w could be less than -1, and built an explicit model. The idea was simple: have a scalar field rolling in a potential, but give it a negative kinetic energy. That means that the field tends to roll up the hill to the top of the potential, rather than rolling down to the bottom. The energy density thus tends to increase, implying wMore recently, Caldwell collaborated with Marc Kamionkowski and Nevin Weinberg on the idea of a "Big Rip." If w is less than -1 and constant, the energy density grows without bound and everything in the universe is ripped to shreds at some finite point in the future. This is a fun idea to think about, but some observers took it too seriously, and began phrasing their limits in terms of how many years would have to pass before there would be a Big Rip. That is just silly; even if wI've gone on too long again. Next time, I promise, I'll talk about my own papers, which are what you really care about, I know. Maybe I'll even talk about what w probably is, in addition to what it is allowed to be.